1. Field of the Invention
This invention relates to a video signal compensator for compensating a video signal for differential picture brightness of an optical image and more particularly relates to a video signal compensator for compensating a video signal for differential picture brightness of an optical image due to uneven illumination. In the preferred embodiment, the video signal compensator compensates for differential picture brightness of an optical image from an endoscope which is brighter at its center than at its periphery. The optical image from the endoscope is imaged onto a video sensor.
2. Description of the Prior Art
A wide variety of optical instruments are used to generate optical images. In the medical field, endoscopes are used in performing surgical procedures, such as minimally invasive surgery, to generate optical images from within a body cavity.
In the industrial field, borescopes are used to inspect interior spaces, such as the interior stage of a jet engine, which are generally inaccessible. Other optical instruments are used for performing such routine tasks as inspecting interiors of sewer lines, ventilation systems, pipe lines and other elongated cavities.
Typically, such optical instruments have a video camera operatively coupled to the proximal end thereof to receive the optical image and to produce an video signal of the optical image. The video signal is typically processed by a video signal processor and displayed on a video monitor or stored by a video storage device.
It is also known in the art that a video sensor may be integral with the proximal end of an optical instrument. The output of the video sensor in such an instrument is typically applied to a video signal processor which processes the video signal to produce a video output signal which is applied to a video monitor. One example of such an instrument is a Video Operating Laparoscope offered for sale and sold by CIRCON ACMI Division of Circon Corporation, the assignee of the present invention.
It is also well known in the art that body cavities, hidden or inaccessible spaces and elongated cavities are either dark or have such low light levels that it is difficult for optical instruments to produce an optical image that can be satisfactorily imaged by a video camera or video sensor.
In order to overcome these problems, a wide variety of light sources have been developed to produce light energy at light levels that provide sufficiently high light levels of illumination in the body cavities, hidden or inaccessible spaces and elongated cavities. A sufficiently high light level enables the optical instruments to produce an optical image of the operative site and to transmit the optical image to the proximal end of the optical instrument enabling the optical image to be imaged by the video camera or video sensor.
In order to accomplish the above, the optical instruments typically include a light guide, such as for example a fiber optic light guide, to transmit light energy from a light source through the proximal end of and through the instrument to the distal end thereof. The light energy is used to illuminate the operative site or area subject to inspection. The optical image is then transmitted by an optical image transferring system from the distal end of and through the instrument to the proximal end thereof where the optical image is directed on the video camera or video sensor.
It is also known in the art that the endoscope may have an illumination source located at the distal end of the endoscope. One of the known prior art laparoscopes had a light bulb located at the distal end of the laparoscope wherein electrical conductors extending from the distal end to the proximal end of the laparoscope energized the light bulb to illuminate the operative site or area subject to inspection. Illumination from the light bulb located at the distal end of the endoscope produced uneven illumination due to the characteristics of the light energy emanating from the light bulb.
It is also known in the art to locate a video sensor, such as a CCD chip, on the distal end of a optical instrument. Such a structure eliminates the use of a optical image transferring system or member. However, such optical instruments still require a light guide to transmit light energy from a light source to the distal end thereof to illuminate the operating site or area subject to inspection. Electrical conductors located within the optical instrument transmit the video signal from the distal tip of and through the instrument to a video signal compensator.
It has been observed that when an optical instrument is used in combination with a light guide, or distally located illumination source, the resulting optical image from the optical instrument has differential picture brightness due to uneven illumination at the distal end thereof.
For example, a typical medical endoscope, having a fiber optic light guide, have diameters generally in the order of about 5 mm to about 10 mm or larger at the distal end. Such endoscopes generally produce an optical image that is brighter in the center and dim on the periphery or edge. In smaller diameter endoscopes having a fiber optical light guide, for example endoscopes having a diameter at the distal end of less that 5 mm, the optical image may be saturated at the center.
One known design approach to solve the differential picture brightness problem is to modify the structure and characteristics of the light guide, the optical image transferring system or member or modify both in an attempt to obtain a more uniform brightness of the optical image developed by the optical instrument itself.
The fiber optic light guides and optics of the optical image transferring systems have been optimized, but, however, the differential picture brightness problem still persists.
The primary cause for the differential picture brightness problem has now been identified to be other than the optical instrument. It has now been identified that it is the light source itself which generates a light energy or light radiation having a peaked characteristic curve with a bright spot in the center thereof and a dim periphery or edge. When the light source is operatively coupled to the light guide, e.g. the fiber optic light guide in an endoscope, the transmitted light energy retains the characteristics of the light source; namely, a bright spot in the center thereof and a dim periphery or edge. In essence, each optical instrument reproduces the characteristic curve of the light source and this results in an optical image having a differential picture brightness due to uneven or non-uniform illumination.
Unsuccessful attempts have been made to design or modify the light source to reduce or eliminate the above described deficiencies.
In addition to the above and as is well known in the art, variations in the operating characteristics of the video sensor or video camera generating the video signals representing optical images introduce shading into the video signal. The combination of the light source problems and shading problems have resulted in poor quality optical images which, in turn, produce poor quality electronic optical images.
It is known in the prior art that vidicon tube cameras, such as for example, a Sony video tube cameras, have used a shading circuit to compensate for the variations in the operating characteristics of the vidicon tube itself (the "Sony Vidicon Shading Circuit"). The Sony Vidicon Shading Circuit used a parabolic waveform and a sawtooth waveform to generate a compensating signal which adjusts the video signal as required to overcome the variations of the vidicon tube operating characteristics.
U.S. Pat. No. 5,343,302 discloses a video camera which includes a correction circuit in which a parabolic wave signal is generated and the level thereof is adjusted in accordance with the zoom and iris settings of the camera's optical system. After adjustment, the parabolic wave signal is clipped in accordance with a reference level and the clipped parabolic wave signal is used for correcting the shading of the cameras image signal. The clipping of the parabolic correction signal allows for a more accurate shading correction. The shading correction on circuit performs the shading corrections principally in the case of the reduction of the light intensity ratio from the periphery to the center of the image caused by aperture eclipse and in the case of f-drop (i.e. reduction of the f-number) at the telephoto lens setting.
U.S. Pat. No. 5,157,497 discloses a method and apparatus for detecting and compensating for white shading errors in a digitized video signal using a flat white calibration target. The correction system is capable of automatically determining the amount of white shading correction to be applied to specific video image pixels as well as the application of that correction to a digitized video signal. The system includes an inspecting portion for identifying the required correction within a video frame, a calculating portion for computing the amount of correction to be applied to the video signal, and a correction portion for correcting the video signal based upon the correction computed by the calculating portion.
U.S. Pat. No. 5,053,879 discloses a method and device for shading correction used in a video printer comprising a TV camera for providing image data of a subject to be printed and an exposure CRT for displaying the image data thereon and to which photographic paper is exposed to make a video printout of the subject. In carrying out the shading correction method, the shading correction device employs a memory for storing the shading correction data, a frame memory for storing image date of a subject to be printed and a device for adding the shading correction data read out from the memory and the image data readout from the memory.
U.S. Pat. No. 4,979,042 disclose apparatus for correcting shading effects in video images for a document retrieval system. The document retrieval system captures an image of a document in electronic form using linear CCD imagers or a CCD array. The apparatus reduces the size of the memory required to store correction information by defining the two dimensional non-uniformity characteristics in terms of two functions that are orthogonal. The orthogonal correction functions are stored in separate memories. During a scan, a pixel counter addresses the X memory while a line counter addresses a Y memory. The correction factors thus obtained are applied sequentially to correct the pixel data value at the current X and Y coordinates. The sources of non-uniformity which are corrected by the apparatus include use of the lens having non-uniformities which are generally known in the optical art as the "cos" law (sometimes known as the cosine law) to focus the image onto the capturing device and, the CCD pixel sensitivity variations and spot uniformities that may occur in an illumination source such as a lamp filament.
The above described prior art represent the typical electronic correction devices and methods to correct shading in an optical image for a variety of video imaging apparatus and system.